Turnover of soil organic matter under pasture as determined by 13C natural abundance

The change in vegetation cover from rainforest with a C3 photosynthetic pathway to grasses with C4 pathways was used to follow input rates and turnover of organic matter in a krasnozem over an 83 year period. The measurement of 613c values on soils from three depths (0.0-7.5, 7.5-15.0, 60.0-80.0 cm) indicated that charcoal was a serious contaminant in the light fractions (1.6 Mg mW3) of all samples and should be removed. Of the two grasses studied (Paspalum dilatatum and Pennisetum clandestinum), the latter gave more input of organic matter into the 7.5-15.0 cm horizon. In the other horizons, both grasses performed equally. Organic matter within microaggregates (0.2 mm) proved to contain up to 32% more old carbon than the remaining soil after 83 years. Turnover times for organic matter in the 1.6 Mg m-3 fraction from the three depths were calculated as 60, 75 and 276 years respectively, compared with 75, 108 and 348 years for the organic matter within microaggregates from the same horizons. It is concluded that the presence of microaggregates is an important factor in stabilizing organic matter in this soil type. Some difficulties with the technique are also discussed.